Cut Phone Line Detector

A while ago I got an email asking for the schematic of a circuit to detect cut phone lines. It didn't take me long to find this circuit in Electronics Now. When the circuit detects that a phone line has been cut, it activates a MOSFET which can be used to drive a relay, motor, etc. It can also be connected to a security system.

Schematic

This is the schematic of the cut phone line detector

Parts

R1,R2,R3____22 Meg 1/4 W Resistor
R4__________2.2 Meg 1/4 W Resistor
C1__________0.47uF 250V Mylar Capicitor
Q1__________2N3904 Transistor
Q2__________2N3906 Transistor
Q3__________IRF510 Power MOSFET
D1__________1N914 Silicon Diode
Load________See "Notes"
MISC________Wire, Phone Connectors, Circiut Board.

Notes

1. The "Load" can be a relay, lamp, motor, etc. The circuit can also be connected to a security system to sound an alarm in case the phone line is cut.

2. If the circuit is connected to a security system or other circuit, both circuits must be electrically isolated from each other using an opto-isolator, relay, etc. This also means that the Cut Phone Line Detector must be powered by a seperate 9V supply.